High-elevation Qilian Mountains and its inspiration for tectonics and biodiversity during the late Middle Miocene

The accurate paleoelevation reconstruction of the Qilian Mountains is critical to advancing our understanding the integrity of the Tibetan Plateau (TP) uplift model, its deep structural mechanisms, and corresponding connections with climatic, environmental, and biodiversity changes. Recently, the first quantitative reconstruction of the paleomidrange (i.e., average elevation of the basin and mountains) of the northern Tibetan Plateau (NTP) was completed, using innovative palynological paleoaltimetry-TP/TPAP [(Tsuga% + Podocarpus%) / (Tsuga% + Podocarpus% + Abies% + Picea%)] ratios, which revealed a rapid uplift of the NTP from low to high elevations during the late Middle Miocene. Here, we analyzed the Ebotu Fauna pollen record (13–12 Ma) from the Hongyazi Basin, situated within the Qilian Mountains, to directly infer the paleoelevation of this region. The pollen assemblages were predominantly composed of conifers (average of 64.0 %), including Picea, Cedrus, and Pinus of the Pinaceae, as well as Cupressaceae, with broadleaves and steppes taxa each accounting for <15 %. This pollen composition evidently reflects a vegetation type dominated by high-mountain conifers forest, consistent with Middle Miocene pollen assemblages from the Qaidam Basin. Application TP/TPAP ratios yielded a paleomidrange of 3492 ± 87 m at 13–12 Ma. Combined with the known elevation of the Qaidam Basin during this period (1885 ± 566 m), the elevation of the Qilian Mountains was calculated to be 4338 ± 653 m. In turn, an elevation of 2646 ± 740 m was obtained for the Hongyazi Basin. This high-elevation terrain provides evidence to support the multi-stage convective removal model of the TP caused by crustal shortening and thickening. It also led to the formation of a humid ecosystem dominated by conifers forest in the Qilian Mountains, supporting the diversification of mammalian taxa.